An artist’s interpretation of the K2-138 system. When they were discovered, these exoplanets gave scientists a window into how planets form when nothing interrupts the process. [Credit: NASA/JPL-Caltech/R. Hurt (IPAC) | Public Domain]
In the last few decades, the study of exoplanets — planets outside our solar system — has exploded. Since the first one was spotted in 1992, scientists have found thousands of different exoplanets in their own unique systems, each of which has told us something new about the cosmos.
Hidden among planets made of diamond and systems that we didn’t think could exist is a wealth of scientific information. To the people that study these strange celestial bodies, finding a “weird one” is a sign that there are still questions to be answered and cosmic investigation to be done. And they are more than ready to start investigating.
Jackie Appel: When people talk about exoplanets, the planets outside our own solar system, they only seem to ask two questions. The big one, of course, is “Could it possibly support life?” But the second question, “How weird is this other world?” can lead to just as much scientific discovery. I hope you’ll come with me to take a look behind the curtain at the study of some of these oddities of outer space.
To date, scientists have discovered 4,292* confirmed exoplanets with help from space telescopes like Kepler and TESS. The vast majority of these planets are pretty average. But what is average when it comes to exoplanets? That is the work of Dr. Michael Meyer from the University of Michigan.
Michael Meyer: What I really want to know and what I think, you know, my kids want to know, and my mom wants to know: is our solar system, or planetary systems like it, common or rare in the Milky Way galaxy? And in order to answer that question, I need to survey the average properties. We’re trying to do the equivalent of the U.S. 2020 census.
Jackie Appel: So what does the average exoplanet look like? Most of them fall into one of three categories: Big, icy planets like Neptune, gas giants kind of like Jupiter or large rocky planets called Super Earths.
But what about the outliers? They must be something special, because the astronomers who study these planets often become very attached to specific discoveries they’ve made, especially if that planet revealed new and unique information. Take Dr. Scott Gaudi from Ohio State University. His pride and joy, Kelt-9b, definitely falls into the new and unique category.
Scott Gaudi: This is the hottest known transiting gas giant planet. So even iron would be in a gaseous form because it’s so hot.
Jackie Appel: With a daytime temperature of 7,800 degrees Fahrenheit, this planet is hotter than some stars. But it was the scientific implications, not the superlative, that interested Gaudi and his team.
Kelt-9b gave researchers insight on how [closely] a planet can orbit a star — it sits so close that its orbit is only about one and a half days long. It also provided a window into the phenomenon of escaping atmospheres, since the atmosphere of Kelt-9b is being slowly blown off the planet by the radiation from it’s too-close star.
Scott Gaudi: We predicted that the atmosphere should be escaping because of the very high amount of high-energy radiation that this planet is being bombarded with. And you can really kind of test these theories that try to predict how much mass loss you should have in these kinds of planets.
Jackie Appel: This process of making a prediction and then checking these extreme planets against the models gives scientists reasons to care about studying these planets, even if we’ll never send a mission there or catch a glimmer of it through a telescope. But not every exoplanet does the work of informing scientists on its own. Some, like the K2-138 system of six planets, need to work together to get their information across.
Jessie Christiansen: If you look at these planets in these systems, they’re exactly telling you their dynamical history. Because there’s only really one way to make these kinds of systems, which is that the planets form while there’s still a disc of material there.
Jackie Appel: Dr. Jessie Christiansen at Caltech and her team of researchers and citizen scientists are credited with the discovery of K2-138. When most solar systems start out, they look like a star surrounded by a disk of gas and debris. Over time, the gas around the star begins to condense and form planets as gravity starts to group ever-growing lumps together. But this process isn’t always smooth. Planets can switch places, or run into each other and break apart, or just be ripped to shreds by the gravity of bigger objects around them. This kind of disruption forms solar systems more like ours, where there’s no real pattern to the spacing or the orbits of the planets.
Jessie Christiansen: So you can only really end up with these really tightly packed resonance systems if you’ve had this very slow, smooth migration with no kicking planets around or in and out or up and down, it has to all happen super smoothly to end up this packed.
Jackie Appel: What K2-138 showed scientists is an example of “perfect” formation; what planetary system formation looks like if nothing interrupts the process. Because of how neatly the planets formed, Christiansen and her team were able to see exactly how they formed, with the innermost planet forming first, the second forming second and so on. The system was so inspiring that the scientists behind System Sounds even turned it into a song.
(System Sounds’ K2-138 song)
Jackie Appel: So what if a discovery comes along that proves the models wrong? That can be just as helpful to scientists. For example, the discovery of what scientists call “Puffy Jupiters,” gas giants that are bigger than they should be given the kinds of gases they’re made of, taught astronomers a lot about how energy and heat can affect the size of a planet, getting bigger as it gets hotter.
Jessie Christiansen: We thought we had a pretty good idea of why Jupiter was the size it was, you know, if you take a bunch of hydrogen and put it at a certain temperature, it should be this big. So when we found these puffy planets that were bigger than we expected, it was like, “Oh, wait, we’re missing something. Like there’s some physics that we haven’t included in our model of how planet interiors work, and planet atmospheres work to account for this.” And that started this really cool period in the field where everyone was like, “What’s going on inside these planets?” Like, what does that mean for the physics?
Jackie Appel: One thing’s for sure. It means more for physics than the superlative alone. Whether it be providing insight into planet formation, showing us why certain planets are so big or letting us probe why they’re so hot, astronomers have learned to never take a weird exoplanet solely at face value.
Scott Gaudi: The biggest, biggest broad brush takeaway from the field of exoplanets so far is that exoplanetary systems are incredibly diverse. Many of them look nothing like our solar system, and many of them are really very extreme and just kind of completely out of the realm of our imaginations.
Jackie Appel: It turns out that the question of “How weird is this other world?” has quite a lot of varied answers. But thanks to these funny little planets floating out there in their strange and unique pockets of space, we’re slowly but surely getting a little closer to actually understanding the universe we live in.
For Scienceline, I’m Jackie Appel.
*This number was correct at the time of reporting. You can find the most up-to-date exoplanet count here.